Digital RF seeks to move beyond hype

SAN FRANCISCO  Digital RF technology has benefited from a move to "digitally-assisted" RF rather than full "digital RF," experts said.

But advocates for digital RF at this year's International Solid State Circuits Conference (ISSCC) such as Bogden Staszewski of Texas Instruments and David Welland of Silicon Laboratories noted that analog portions of the RF signal processing chain which may be superseded by digital circuitry.

Skeptics see little value in the process of replacing analog RF circuitry with less efficient digital circuitry. At their heart, cellphone, WLAN and WiMAX transmissions ride on a radio frequency carrier. A series of digital filters may strip the carrier of harmonics and spurs while digital modulators attempt to symbolize data bits with elaborate variations in patterns of phase, frequency and amplitude.

ISSCC panelist Asad Abidi of UCLA noted the disparities between analog and digital components. Supplementing a traditional super heterodyne receiver with digital components and then converting to digital reduces power consumption to roughly 10 mW. On the other hand, Abidi said, displacing analog components with a direct conversion architecture increases power consumption to tens of watts.

Infineon Technologies' engineering chief Rudolf Koch, said his company is more open to the use of digital in the RF signal processing chain. The only way to address the multiplicity of 2G, 3G and 4G transmission standards, he said, was to utilize what he called an "intelligent PA," or power amplifier. To do that, the A/D converter must be moved closer to the antenna to utilize higher integration levels for the transmitter-receiver blocks.

But certain parts of the RF receiver chain cannot be rendered in digital CMOS, Koch said. These include analog front-end components. "Operating below one volt, you'd die from the noise," he said.

Intel's Krishnamurthy Soumyanath and TI's Staszewski both made the case for replacing everything in the signal processing chain with digital circuitry. Digital circuitry supports the goal of a software-defined radio, they said, explaining that the goal is more than an "integrated radio," according to Soumyanath. The goal is "cognitive radio," he added. Analog signal processing remains a "boat anchor," Soumyanath insisted, because it does not scale with digital baseband processors.

Staszewski claims to have built an all-digital transmitter. Amplitude was adjusted by adding or subtracting charge pumps, Staszewski explained, adding that the final component needed is a transistor with a 250 GHz fT.

With most of the ISSCC panelists agreeing that digital RF will prove useful, it's possible the technology will advance beyond promotional hype.

Stephan Ohr is research director for analog semiconductors at Gartner Dataquest Research.